The present invention relates to an improved process for the commercial production of malononitrile.
Malononitrile is a versatile compound of exceptional reactivity that makes it one of the most important organic intermediates used in research and in the chemical industry. It is a keystone in the syntheses of pharmaceuticals, dyestuffs, pesticides, fungicides and a variety of polymers.
Malononitrile has been commercially produced by the reaction of cyanogen chloride with acetonitrile in a cylindrical reactor at about 750xc2x0 C. There are certain disadvantages associated with this process: first, the high operating temperature requires a heat-resistant apparatus that is expensive to construct and to operate; and secondly, the malononitrile reaction product is contaminated with by-products such as carbon and polymers that are difficult to separate and which increases the cost associated with the product.
Malononitrile has also been produced commercially by a process that includes the dehydration of cyanoacetamide with phosphorous pentachloride and other phosphorous compounds. However, a major drawback of this process is that it produces relative large quantity of phosphate waste as a by-product.
Japanese patent publication 57 203051 discloses that malononitrile can be prepared by treating cyanoacetaldehyde dimethyl acetal, NCCH2CH(OCH3)2, with an aqueous solution, followed by reaction with hydroxylamine-O-sulfonic acid, H2NOSO3H. However, the disposal of the acid waste generated by this process would constitute a major drawback to its commercial use.
It is known from Olah et als."" publication in Synthesis (1980, 657-58) that cyanuric chloride is useful as a mild dehydrating agent in the preparation of nitrites from amides when the reaction was carried out in N,N-dimethylformamide as a solvent. However, the method as disclosed by Olah et al. does not lend itself to the commercial production of malononitrile due to the fact that the N,N-dimethylformamide has a relatively high boiling point and an inclusion complex, or adduct, is formed during the separation step when the temperature exceeds about 100xc2x0 C. Thus, when the mixture of the highly reactive malononitrile and N,N-dimethylformamide reaches about 100xc2x0 C. the compounds react to form the adduct which prevents further malononitrile from being isolated and recovered from the reaction mixture. As a result, the relatively low yield of malononitrile and processing and disposal problems associated with the adduct by-product [c.f., Moetz and Rodriguez, Tetra. Letters, Vol. 38, No. 24, pp. 4221-22 (1997)] renders the dehydration process as described by Olah et al. of no commercial significance.
It is therefore an object of this invention to provide an improved process for the commercial production of malononitrile that can be carried out under mild conditions, i.e., in a process that does not require extremes of temperature and/or pressure.
Another object of the invention is to provide a novel process for the production of malononitrile in improved yields and without the production of by-products, the disposal of which are difficult and expensive.
Yet another object of this invention is to provide an improved commercial process in which the malononitrile end product is easily separated from the reaction mixture.
What has been found is that N,N-dimethylformamide functions in relatively small amounts and under mild conditions as a dehydration catalyst when cyanuric chloride is employed as a dehydration agent for cyanoacetamide.
In accordance with the invention, malononitrile is synthesized by reacting cyanoacetamide and cyanuric chloride in the presence of a catalytic amount of N,N-dimethylformamide (xe2x80x9cDMFxe2x80x9d) in accordance with the reaction scheme. 
The reaction is preferably carried out in a polar solvent in which the cyanoacetamide is readily soluble. A preferred polar solvent is acetonitrile. Suitable polar solvents in addition to acetonitrile include tetrahydrofuran, 1,4-dioxane, and ethylacetate.
The reaction is conducted at a temperature in the range of from 10xc2x0 C. to 100xc2x0 C., and preferably in the range of from about 50xc2x0 C. to about 80xc2x0 C., and most preferably in the range of from about 50xc2x0 C. to about 60xc2x0 C.
The advantages of this process are as follows: first, the dehydration reaction is carried out under relatively mild conditions of temperature and pressure; second, malononitrile is synthesized in good yields and can easily be separated from the reaction mixture; and third, there is no phosphate waste or other by-product generated by the process. Moreover, the process of the invention has the additional benefit that the DMF can be recovered for reuse.
This reaction scheme avoids the drawbacks of the process disclosed by Olah et al. by the use of a polar solvent for the cyanoacetamide that (1) has a relatively low boiling point to facilitate its removal and recovery for reuse; and (2) does not react with the malononitrile, thereby increasing the yield of the desired product.
In accordance with the novel process of the invention, malononitrile is synthesized by reacting the equivalent of one mole of cyanoacetamide and 0.42 mole-equivalents of cyanuric chloride in the presence of a catalytic amount of N,N-dimethylformamide. The DMF can be present in the range of from about 0.05 mol to about 0.30 mol, and preferably in a range from about 0.10 mol to about 0.20 mol, based on one mole of cyanoacetamide, the most preferred amount being 0.16 mol per mole of cyanoacetamide, for practice of the process under the conditions of temperature and pressure identified above and in the examples which follow. Thus, the optimum molar equivalents are 0.42 mol of cyanuric chloride and 0.16 mol of N,N-dimethylformamide per mole of cyanoacetamide.
The reaction is carried out in a polar solvent, preferably acetonitrile, in accordance with the reaction scheme of equation (I), above. The reaction mixture is maintained at a temperature in the range from 10xc2x0 C. to 100xc2x0 C., and preferably in the range from 50xc2x0 C. to 80xc2x0 C., and most preferably in the range from 50xc2x0 C. to about 60xc2x0 C.
In a preferred embodiment, the cyanoacetamide is first dissolved in the solvent, which step, for convenience, can be completed by adding the solvent, e.g., acetonitrile, to the reaction vessel and adding the crystalline cyanoacetamide to the acetonitrile solvent with stirring. Since both compounds are flammable and the acetonitrile, or methyl cyanide, is toxic by skin absorption, the reaction is conducted under a nitrogen atmosphere in accordance with well-known standard industry practice.
The cyanoacetamide solution is maintained at a temperature in the range of about 50xc2x0 C.-60xc2x0 C. by a heated water jacket or a thermostatically controlled electrical heater.
After the cyanoacetamide is completely dissolved in the solvent, the cyanuric chloride is added slowly to the reaction vessel, the contents of which are continuously stirred. Simultaneously, the catalyst N,N-dimethylformamide is slowly added to assure thorough mixing of the reactants. The addition of the second reactant and proportional addition of the catalyst is preferably completed over a period of 5 to 7 hours. The reaction is exothermic and the temperature of the stirred reactants rises and should not be allowed to exceed a temperature of 100xc2x0 C. in the vessel in order to avoid decomposition of the product. Hydrogen chloride gas generated during the reaction is removed by any convenient means, e.g., absorption in a base trap containing caustic.
After catalyst addition is complete, the contents of the vessel are stirred for approximately another five (5) hours while maintaining the temperature in the range of from about 50xc2x0 C. to 60xc2x0 C. After addition of the catalyst and cyanuric chloride has been completed, the progress of the reaction is monitored by gas chromatographic (xe2x80x9cGCxe2x80x9d) analysis.
When the reaction has proceeded to the desired degree of completion as determined by GC, the reaction mixture is filtered, the precipitate being cyanuric acid which is recovered and air dried . This material can be used as an antifouling agent and has utility as a marine biocide. The recovery of this material as a by-product having industrial utility, rather than a waste stream requiring treatment for environmentally acceptable disposal, is another commercially important aspect of the invention.
The filtrate is concentrated, as by heating under vacuum at about 200 mm Hg and the malononitrile is recovered in purified form by vacuum distillation. The yield, which is dependent upon the solvent employed, will be in the range of from about 50% to about 75% in the commercial-scale practice of the process.